The present invention relates to a method and to apparatus for demodulating a signal in which a carrier is modulated with M phase states, M being an integer, and generally even, and in particular M=2.sup.m states (m being an integer).
A particularly important, though non-exclusive application of the invention lies in digital transmission systems using quaternary phase shift keying or 8-state phase shift keying (QPSK or 8PSK) or modulation of a microwave carrier (i.e. having a frequency of at least 3 GHz). A particular application that may be mentioned is simultaneous transmission of a plurality of digital television programs in the form of a multiplex in a satellite broadcast channel in the 12 GHz band.
Nevertheless, as can be seen below, the invention applies to demodulating any signal using PSK modulation of a carrier and it is advantageous whenever carrier recovery conditions are unfavorable.
Systems for transmission and broadcasting over a satellite channel often use PSK type modulation (in particular QPSK or 8PSK) with prior encoding for protection against errors that increases the required data rate. In particular, use is made of convolutional error protection codes or of concatenated codes (associating convolutional encoding with Reed-Solomon encoding), thereby eliminating errors almost completely even for Eb/N.sub.0 ratios as small as 2 dB to 4 dB (where Eb is energy conveyed per bit and N.sub.0 is noise level). The efficiency of encoding systems causes these values to correspond to Ec/N.sub.0 ratios that are close to 0 dB (where Ec is energy per bit transmitted after encoding).
The same problem is encountered when using trellis modes of encoding which enable the spectrum efficiency of a non-coded system to be retained by increasing the number of modulation states.
Error protection encoding systems increase the data rate to be transmitted for a given number of useful bits, but complicate carrier recovery when demodulating on reception, and very often it is carrier recovery performance that puts a limit on the overall performance of the system.